1. Field of the Invention
The present invention relates to logic differential logic/buffer circuits, and more particularly to current transfer logic circuits incorporating fail-safe circuitry.
2. Background Information
Fail safe differential amplifiers or receivers provide a known output when an indeterminate or invalid input signal is present at the inputs. An invalid input signal generally occurs when the inputs are floating or three-stated, or shorted. But partial shorts or opens may lead to invalid input signals. In the face of such invalid input signals the receivers usually will oscillate, switch on noise or will be in an indeterminate state.
Prior solutions have addressed voltage based and low voltage based circuits. These solutions have provided bias resistors at the inputs of the differential receiver to bias the inputs to a known condition by providing a dc offset at the inputs. But, such an offset may unbalance return currents, distort the output and possibly load and reduce the input signal amplitude. Other solutions have biased the inputs of the receiver to Vcc with logic to drive the receiver output to some known state.
Another solution for low voltage based circuits is found in Texas Instruments differential receiver, part no. SN65LVDT32B, and several other similar devices. The circuit of this device provides two active circuit high impedance comparators sharing the receiver inputs. These comparators provide a window with one comparator providing a +80 millivolt threshold and the other a −80 millivolt threshold. A fail-safe timer is “anded” with the comparator outputs and if the differential input is within the +/−80 millivolt window at the end of the timer period, the output is driven to a known fail safe high state. One limitation of this circuit is that the fail-safe timer must switch to start the time period. If the input to the receiver is valid, say more than +80 millivolts differential, but then reverts to an invalid state, say +10 millivolts differential, the timer may not be started since the receiver output may not switch.
Yet another fail safe device for low voltage circuits is produced by Maxim, part no. MAX9153/4. This device is labeled as a repeater, but in fact is a differential amplifier or receiver circuit. This circuit has diode spike suppressors and may not operate when powered up with the transmission line shorted, or with low level (under 100 millivolts) attenuated differential signals. High frequencies operation may be also impaired.
An objective of this invention is to provide an active failsafe receiver circuit that takes the output of the receiver to a stable known state if any of the following is true.                1. Receiver inputs are floating and unterminated.        2. Receiver inputs are terminated and undriven due to the driver being unpowered, disabled and/or disconneted.        3. The input cable is cut.        4. Receiver inputs are shorted together, say due to driver outputs being shorted, or one or both receiver inputs are shorted to ground due to the driver outputs puts being shorted to ground or there is a short in the cable.        
None of these above conditions will produce an indeterminate output from the present invention and while under normal operation, the failsafe bias will not affect the receiver performance from both speed (bandwidth) and/or a jitter/noise point of view.
Another objective of the present invention is to provide a fail-safe current mode receiver that is competitive in power and chip area.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to illustrative embodiments, the drawings, and methods of use, the present invention is not intended to be limited to these embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be defined as only set forth in the accompanying claims.